Inorganic oxides such as titanium dioxide are used as pigments in connection with paint, paper, polymer compositions and other products. Such pigments are generally produced in powder form with specific properties and characteristics (for example, specific particle sizes, shapes and lattice structures). The pigment particles are typically coated with one or more materials to modify or enhance the properties and characteristics of the pigment for particular applications. For example, the pigment particles are often coated with compounds that function to improve the opacity, brightness, light stability, durability and light scattering properties or tint strength (hiding power) of the pigment.
An inorganic oxide that is very popular for use as a white pigment is titanium dioxide. Titanium dioxide can be manufactured by either the sulfate process or the chloride process.
In the sulfate process for manufacturing titanium dioxide, a titanium slag ore, usually an ilmenite, is dissolved in sulfuric acid to form a mixture of sulfates, including titanyl sulfate. Iron is removed from the solution. The titanyl sulfate is then hydrolyzed in solution to yield insoluble, hydrated titanium dioxide. The hydrated titanium dioxide is heated in a calciner to evaporate the water and decompose the sulfuric acid in the solid. The solid is then converted into seed crystals which can be milled to the desired size.
In the chloride process for manufacturing titanium dioxide, a dry titanium dioxide ore is fed into a chlorinator together with coke and chlorine to make a titanium halide (such as titanium tetrachloride). Streams of gaseous titanium halide (such as titanium tetrachloride) and oxygen are heated and introduced at high flow rates into an elongated vapor phase oxidation reactor conduit. A high temperature (approximately 2000° F. to 2800° F.) oxidation reaction takes place in the reactor conduit whereby particulate solid titanium dioxide and gaseous reaction products are produced. The titanium dioxide and gaseous reaction products are then cooled, and the titanium dioxide particles are recovered.
A potential problem associated with inorganic oxide pigments such as titanium dioxide pigments is the tendency of the pigment particles to flocculate or agglomerate in the base medium (i.e., the slurry of the paint, paper, polymer composition or other material in which the pigment is incorporated). Flocculation or agglomeration of the pigment particles can adversely impact desirable properties of the pigment including the opacity, brightness and light scattering efficiency of the pigment.
A related problem that can result from a high concentration of the pigment in the base medium is what is referred to as optical crowding. For example, optical crowding can occur when a high concentration of the pigment is incorporated into a polymer composition. When the concentration of the pigment in the base medium increases to a certain level, the light scattering efficiency or tint strength of the pigment can substantially decrease. The light scattering cross-section of an inorganic oxide pigment particle is significantly greater than the actual cross-section (area) of the pigment particle. At high pigment concentrations, the pigment particles are closer to one another which results in an overlap of the respective light scattering cross-sections of the particles and thereby reduces the light scattering efficiency of the dispersed pigment. In addition to the light scattering efficiency of the pigment, the optical crowding effect can also decrease the light stability, brightness and opacity of the pigment.
Various techniques have been utilized in an attempt to prevent inorganic oxide pigment particles from flocculating and agglomerating and to diminish the optical crowding effect. For example, the pigment particles have been coated with various inorganic compounds that function to modify the surface charges of the particles and impart other properties to the particles. Also, spacers, fillers and extenders have been utilized to space adjacent particles apart from one another. For example, spacer particles can be formed on the surfaces of the pigment particles in situ in an aqueous slurry containing the pigment particles. Examples of spacers, fillers and extenders that have been utilized include clay, calcium carbonate, alumina, silica and other metal oxide compounds. Metal oxide particles such as zirconia and titania can also be used, although such materials may be cost prohibitive for use on a commercial scale.
Although such techniques have been utilized with varying degrees of success, there is still room for improvement. For example, spacer particles can be difficult to disperse in the base medium and actually decrease the concentration of titanium dioxide particles that can be utilized. Also, many dry hide grade pigments that have been processed to have improved light scattering properties have relatively low bulk densities and are fluffy which makes them difficult to handle, bag and transport. For example, filling a bag or other container to capacity with a treated titanium dioxide pigment can be difficult to accomplish in an efficient manner without first deaerating the pigment. In a continuous manufacturing and packaging process, the additional time and handling required to fill the container to capacity and impart a consistent, predetermined amount of pigment to each bag can make the process inefficient.
In order to overcome the problem associated with the low bulk density of the pigment, some dry hide grade pigments are supplied in the form of slurries having high (65-75% solids concentrations). The use of slurries, however, can be problematic in some applications.